Porosity Evolution and Mineral Paragenesis During Low-Grade Metamorphism of Basaltic Lavas at Teigarhorn, Eastern Iceland Philip S
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[AMERICAN JOURNAL OF SCIENCE,VOL. 299, JUNE, 1999, P. 467–501] POROSITY EVOLUTION AND MINERAL PARAGENESIS DURING LOW-GRADE METAMORPHISM OF BASALTIC LAVAS AT TEIGARHORN, EASTERN ICELAND PHILIP S. NEUHOFF*, THRA´ INN FRIDRIKSSON*, STEFA´ N ARNO´ RSSON**, and DENNIS K. BIRD* ABSTRACT. Low-grade alteration of basaltic lavas at Teigarhorn, eastern Iceland, resulted in three distinct stages of mineral paragenesis that correlate to events in the burial and intrusive history of Icelandic crust. Metasomatism and brittle deformation during the paragenetic stages dramatically affected the paleo- hydrology of the lavas and formed temporally distinct mineral assemblages. The lavas initially contained up to 22 percent total porosity concentrated near the tops and bottoms of individual lava flows. Celadonite and silica (Stage I) precipi- tated along the walls of primary pores prior to deep burial of the lavas and occluded ϳ8 percent of the initial porosity. During burial (Stage II), hydrolysis of olivine and basaltic glass led to the formation of mixed-layer chlorite/smectite clays in the matrix of the lavas and as rims filling ϳ40 percent of the volume of primary pores. Chlorite contents in Stage II mixed-layer clay rims increased from ϳ20 to ϳ80 percent during the infilling of individual vesicles, reflecting increas- ing temperatures with time as the lavas were buried. The end of Stage II occurred after burial and is represented by filling of ϳ40 percent of total primary porosity -by zeolites (scolecite or heulandite ؉ stilbite ؉ mordenite ؎ epistilbite) and re placement of plagioclase by zeolites and albite. The Stage II zeolite assemblages are indicative of two regional metamorphic mineral zones in eastern Iceland, the -mesolite ؉ scolecite and heulandite ؉ stilbite zones. The presence of the bound ary between the mesolite ؉ scolecite ؉ and heulandite ؉ stilbite zones indicates .that the maximum temperature during burial metamorphism was 90° ؎ 10°C Localized areas of intense hydrothermal alteration associated with intrusion of basaltic dikes (Stage III) overprint Stages I and II. Extensive fracturing and hydrothermal brecciation during Stage III added 3 to 11 percent total porosity in which mm- to cm-scale museum-grade crystals of quartz, calcite (Iceland spar), stilbite, scolecite, heulandite, and laumontite precipitated. Estimates of the tem- perature during Stage III (based on fluid inclusion homogenization temperatures in calcite, chlorite geothermometry, and the zeolite assemblage) range from 120° to 180°C. Although the thermobarometric conditions during Stages II and III led to similar mineral assemblages, careful attention to textural and geologic rela- tions permits seemingly complex, multi-stage parageneses in metabasalts to be interpreted in a petrotectonic context. INTRODUCTION The mineralogy of low-grade metabasalts is a sensitive indicator of the thermobaro- metric evolution of oceanic crust and continental flood basalts. Discrete temperature-, pressure-, and/or depth-controlled zones characterized by assemblages of silica miner- als, phyllosilicates, and zeolites frequently serve as metamorphic indicators in low-grade metabasalts. Zones containing trioctahedral smectite, corrensite and/or mixed layer chlorite/smectite, and chlorite occur with increasing metamorphic grade in zeolite- and greenschist-facies metabasalts and active geothermal systems in basaltic rocks (Mehegan, Robinson, and Delany, 1982; Seki and others, 1983; Liou and others, 1985; Bevins, Robinson, and Rowbotham, 1991; Robinson, Bevins, and Rowbotham, 1993; Schmidt, 1990; Schmidt and Robinson, 1997; Kristmannsdo´ ttir, 1979; Schiffman and Fridleifsson, 1991; Robinson and Bevins, 1999; Alt, 1999). In addition, as many as five separate depth-controlled ‘‘zeolite zones’’ have been described in regionally metamorphosed basaltic lavas and Icelandic geothermal systems (Walker, 1951, 1960a,b; Sukheswala, * Department of Geological and Environmental Science, Stanford University, Stanford, California 94305-2115 ** Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavik, Iceland 467 468 Philip S. Neuhoff and others—Porosity evolution and mineral Avasia, and Gangopadhyay, 1974; Kristmannsdo´ ttir and To´ masson, 1978; Jørgensen, 1984; Murata, Formoso, and Roisenberg, 1987; Schmidt, 1990; Neuhoff and others, 1997; Christiansen and others, 1999). Phyllosilicate and zeolite parageneses thus provide useful information for evaluating heat flow and structural evolution in large igneous provinces (Neuhoff and others, 1997) and exploration potential in fractured basaltic petroleum reservoirs (Christiansen and others, 1999). Complicating such interpretations, however, is the fact that basaltic provinces often undergo multiple stages of intrusion, extrusion, and deformation during which the lavas are altered through weathering, regional burial metamorphism, and local hydrothermal/contact metamorphism. Min- eral assemblages formed during regional metamorphism and localized hydrothermal alteration can be nearly identical, complicating interpretation of mineral isograds associated with burial metamorphism (Neuhoff and others, 1997; Sukheswala, Avasia, and Gangopadhyay, 1974). In the present study, we demonstrate that regional and local mineral parageneses in low-grade metabasalts can be distinguished on the basis of geologic and textural relations. These relations are difficult to establish from drillcores and areas of limited outcrop. Ideally one would like to investigate the textural and geologic relations between mineral parageneses in laterally continuous, unweathered outcrop exposures. Remark- able outcrop exposures created through coastal erosion at Teigarhorn, eastern Iceland (fig. 1), permit such detailed observations of geologic and textural relationships in basaltic lavas affected by burial metamorphism, brittle deformation, and hydrothermal alteration during dike emplacement. Geologic, petrographic, mineral chemical, and hydrologic information based on field and laboratory studies are presented in order to distinguish mineral assemblages and textures associated with three paragenetic stages of alteration at Teigarhorn: syn-eruptive, near surface alteration (Stage I), regional burial metamorphism (Stage II), and local hydrothermal alteration associated with dike emplace- ment (Stage III). Each stage is characterized by distinct mineral parageneses and textures, with Stage II resulting in formation of the regional zeolite zones described by Walker (1960b). Bulk porosity varied considerably during the metamorphic history of the lavas due to filling of pore spaces by secondary minerals and generation of secondary porosity through brittle deformation. The results provide an example of how observa- tions of mineral paragenesis and porosity evolution can be integrated in a petrotectonic interpretation of low-grade metamorphism in basaltic terraines. GEOLOGICAL SETTING Geology.—Teigarhorn is located near the eastern edge of the volcanic plateau of present-day Iceland (fig. 1). Lavas erupted from fissures and central volcanoes during magmatism along a divergent plate boundary between the North American and Eur- asian plates (Kristja´nsson, Gudmundsson, and Haraldsson, 1995). The oldest exposed lavas are only about 13 Ma (Watkins and Walker, 1977). Upper crustal stratigraphy in eastern Iceland consists largely of subaerial basaltic lavas with minor intercalated sediments, silicic lavas, lignite layers, and pyroclastic units (Robinson and others, 1982; Gu´ stafsson, 1992; Walker, 1960b; Wood, 1977; Sæmundsson, 1979). Continued volca- nism and rifting led to pronounced tilting of the lavas that dominate crustal structure around Teigarhorn (Pa´lmason, 1973); dips increase from Ͻ2° to 6° southwest at the top of the lava pile to ϳ6° to 9° southwest at sealevel (fig. 1; Walker, 1974). Disrupting the monoclinal structure of the lava pile are mafic and silicic intrusive centers and central volcanic complexes (fig. 1; Walker, 1963). Extrusive products of the extinct central volcanoes are interlayered with fissure-erupted lavas that make up most of the crust in eastern Iceland. Glacial exhumation has revealed an alignment of mafic dice swarms, central volcanoes, and intrusive centers that defines the geometry of paleo-rift zones. Teigarhorn lies within a prominent dike swarm (called the A´ lftafjo¨ rdur dike paragenesis during low-grade metamorphism of basaltic lavas at Teigarhorn 469 Fig. 1. Generalized geological map of a portion of eastern Iceland (after Jo´ hannesson and Sæmundsson, 1989) showing the distribution of major rock types, locations of intrusions and central volcanoes, and selected lava orientations from Walker (1963, 1974). Dashed curve outlines the A´ lftafjo¨ rdur mafic dike swarm where dikes make up greater than 8 percent (by volume) of the crust (Walker, 1963). swarm; Walker, 1963): the A´ lftafjo¨ rdur volcanic center and the Austurhorn intrusion both lie within this swarm (figs. 1 and 2; Walker, 1963; Blake, 1970). The composition and mineralogy of these dikes are similar to the surrounding basalts discussed below (Walker, 1963). The dikes are subvertical, trend north-northeast (fig. 2), and vary in thickness from 0.25 to 3 m. Lithology.—The volcanic stratigraphy exposed at Teigarhorn includes approx 150 m of subaerial mafic lava flows with minor intercalated pyroclastic units. The uppermost Fig. 2. Map of Teigarhorn showing the distribution of individual mafic dikes (determined by field mapping and from aerial photographs) and the locations of samples and porosity measurements.